Marcum, who has not pitched in a Grapefruit League game in nearly two weeks because of shoulder tightness and neck discomfort, would join fellow starting pitching Johan Santana on the DL to open the season.
Mets manager Terry Collins said left-hander Aaron Laffey, who was dispatched to minor league camp Sunday, likely will break camp with the Mets as a reliever and then as the fifth starter when one is required.
"Laffey is our No. 1 option right now," Collins said.
Marcum is due for more tests on his neck Wednesday and is being treated with anti-inflammatory medication. He received a cortisone shot in his pitching shoulder last Tuesday.
"He hasn't pitched in two weeks," Collins said. "I mean, to have him all of a sudden ready to pitch Game 3 or 4, I'd be surprised."
A Mets official told ESPNNewYork.com that the Mets very likely would handle any rotation issues internally and not seek outside help such as Chris Young.
Collins also ruled out top prospect Zack Wheeler being a consideration.
Laffey, 27, had tossed nine scoreless Grapefruit League innings before getting roughed up Sunday in a split-squad game against the St. Louis Cardinals. He allowed four runs on six hits and two walks in three innings.
Laffey was 4-6 with a 4.56 ERA in 22 appearances [16 starts] for the Toronto Blue Jays last season.
Collins said Laffey would work in the bullpen until a fifth starter is required, which would be the team's sixth game, on April 7 against the Miami Marlins.
But Collins said there may be merit in using Harvey to separate Gee and Hefner in the rotation because of their similar styles. The order beyond Jonathon Niese on Opening Day will be resolved later this week.
"If we bring Matt in Game 2, then you've got Gee and Hefner back-to-back," Collins said. "Do you want those guys pitching back-to-back against the same club? They won't have to this [first] time. That stuff is going to be decided on Thursday."
Hefner avoided a major injury Tuesday. He suffered a bruise on the side of his right elbow when struck by a one-hop comebacker off the bat of Carlos Beltran in the fourth inning. X-rays of Hefner's elbow came back negative.
Collins expects Hefner will be available to open the season.
"Hopefully it's a maximum of just a couple of days to get the swelling out of it and start throwing again and see where we're at," Hefner said. "It's not actually on the bone, because it was bent when it happened. So it got a little more muscle than the bone.
"It already feels better. That's a good sign. And my strength is back in my hand -- because right after, the trauma of banging, I couldn't really grip. But that's all come back and it seems to be progressing already."